Radiation surgery has been successfully employed to treat tissue. Radiation surgery involves the application of radiation to tissue to remove or alter the condition of the tissue. Radiation can be generated in the form of light from a laser or a lamp such as a flash lamp, or heat from an RF source. Alternatively, radiation can be generated in the form of microwaves or ultrasound.
Laser surgery has been successfully employed to remove hair and to treat skin abnormalities such as vascular lesions. For example, in performing a laser surgery, a beam of laser light having a selected wavelength is applied to a targeted region of the skin to selectively destroy the cutaneous blood vessels or melanin depending on the application. When removing unwanted hair, a beam of laser light is applied to the targeted skin. The light penetrates deep into the dermal tissue region, where the light is absorbed by peri-follicular melanin, reaching the follicle, bulb, bulge, and vascular supply to eliminate unwanted hair and impede its growth. In treating vascular lesions such as a port wine stain, laser light is preferentially absorbed by the hemoglobin which is the major chromophore in the blood in the ectatic capillaries in the upper dermis. The light energy is converted to heat, causing thermal damage and thrombosis in the targeted vessels.
Laser treatments, however, can be painful to a patient. To reduce pain, the cooling of tissue has been employed during laser treatment. U.S. Pat. No. 5,814,040, incorporated herein by reference, describes cooling an epidermal tissue region while performing selective photothermolysis of selected buried chromospheres in biological tissues using a laser. This cooling procedure is known as dynamic cooling. In this procedure, an epidermal tissue region is cooled by spraying with a cryogen to establish a predetermined dynamic temperature profile. The epidermal and underlying dermal tissue regions are subsequently irradiated to thermally treat the dermal tissue region while leaving the epidermal tissue region substantially undamaged. Cooling the epidermal tissue region reduces pain suffered by the patient during the procedure, and permits application of higher dosage radiation.
The GentleLase.TM. laser treatment system for hair removal and treatment of vascular lesions manufactured by Candela Corporation (Wayland, Mass.) employs dynamic cooling technology. The GentleLase.TM. laser treatment system includes a control unit and a handpiece. The control unit includes a flashlamp excited long-pulse alexandrite laser, a source of HFC 134a liquid cryogen, and electronics for controlling the system. The handpiece receives the laser light and the cryogen from the control unit through a cable which includes an optical fiber, wires, a delivery tube and an electronically controlled valve. The handpiece delivers the cryogen and the laser to tissue being treated. In the procedure, a highly focused spray of HFC 134a cryogen is applied on the patient's skin for 20-100 milliseconds, and after waiting 0-3 milliseconds, the laser pulse is applied to the patient's skin.
Clinical tests have shown that in order to achieve a desirable low temperature profile (e.g. -30.degree. C. to 25.degree. C.) in the epidermal tissue region, a waiting period after applying the cryogen of up to about 250 milliseconds is needed before applying the laser pulses. The desirable low temperature profile can vary depending on the skin tone of the patient and the objective in cooling. Only a few degrees below normal skin temperature may be sufficient when treating a patient having a light skin tone. On the other hand, when treating a patient having a dark skin tone, cooling to -30.degree. C. may be desired. One problem encountered during dynamic cooling of an epidermal tissue region with a waiting period of up to about 250 milliseconds is that a bright light flash not associated with normal laser treatment has been observed during the procedure. The bright light flash resembles a flame, and tends to frighten the patients and interfere with light transmission. Therefore, a shorter waiting period of about 3 milliseconds which may be insufficient to obtain an optimal low temperature profile, is presently used to suppress the abnormal light flash. It is expected that similar flashing problems can occur during radiation treatment using other radiation sources.